1. Field of the Invention
The present invention relates to a brushless direct current monophasic motor, and more particularly to a drive circuit thereof.
2. Description of the Related Art
In office automation equipment such as personal computers or photocopiers, a number of electronic devices have electronic components, which are contained in a relatively narrow housing. There is a concern that the heat generated from the above-mentioned electronic components is confined in the housing and destroys the electronic components by heat. Therefore, air vents are made and a fan motor is mounted to the air vent to evacuate the heat from inside the housing to the outside.
For this purpose, it is not uncommon to use a brushless direct current monophase motor. FIG. 3 describes a drive circuit of the past for such a brushless direct current monophasic motor.
In this figure, +B represents the direct current power source to drive a brushless direct current monophase motor coil (motor coil) L.
The drive circuit is constituted by mounting 4 switching elements, here, MOS type power FETs (field-effect transistors) Q1-Q4, a diode D1 and an electrolytic capacitor group 31.
The DC—DC converter 34 is a voltage conversion circuit that supplies a direct current power source, which is below the voltage of the direct current source +B, to a pre-drive circuit 35.
The pre-drive circuit 35 is a circuit that receives signals from a motor rotation position detector 36 and a duty ratio controller in PWM (pulse width modulation), supplies suitably set gate signals GS1-GS4 to FETs Q1-Q4, and controls their ON/OFF states. In addition, 32 is a fuse, and 33 is a filter circuit.
The motor coil L is mounted on the stator of the motor (not shown) and energized by the FETs Q1-Q4 with a prescribed ON/OFF timing, to generate a dynamic magnetic field (rotating magnetic field).
A permanent magnet is mounted on the rotor of the motor (not shown), and the rotation of the permanent magnet by tracking of the above-mentioned magnetic field rotates the rotor.
The first embodiment (shown in FIG. 1) will be explained with the assumption that the normal rotation state is obtained when energizing from the left end to the right end of the motor coil L. The assumption is discussed below with respect to the prior art example shown in FIG. 3.
First, it is assumed that the gate signals GS1-GS4, which induce rotation with a 100% duty ratio from the left end to the right end of the motor coil L in the figure, are output from the pre-drive circuit 35 to the FETs Q1-Q4 (gates G1-G4). In this case, high-level gate signals GS1 and GS4 are output to FETS Q1 and Q4, and low level gate signals GS2 and GS3 are output to FETs Q2 and Q3.
At this moment, the voltage for the high level gate signal GS1 to FET Q1 is applied up to a prescribed level that exceeds the voltage of the direct current power source +B, setting FET Q1 to ON. The high-level gate signal GS4 is provided to FET Q4 with a voltage level that remains sufficient to set FET Q4 to ON.
Meanwhile, since low level gate signals GS2 and GS3 are output to FETs Q2 and Q3, FETs Q2 and Q3 are all in the OFF state.
Therefore, at each high level period of gate signals GS1 and GS4 to FETs Q1 and Q4, the current from the direct current power source +B flows through the following path: diode D1→Q1 (between the drain and the source)→motor coil L→FET Q4 (between the drain and the source)→ground.
The rise and fall of the gate signals GS1 and GS4 of FETs Q1 and Q4 are always simultaneous as long as the duty ratio is set to 100%, and the motor (rotor) rotates at maximum speed.
Next, to perform rotation in case the duty ratio is decreased to less than 100%, for example to 50%, regarding only the falling timing of the gate signal GS 4 to FET Q4, the time point is earlier by only ½, compared to that at 100% described above.
Accordingly, if the duty ratio is only 50%, the amount of time the current flows through the direct current power source +B to the diode D1→FET Q1→motor coil L→FET Q4→ground, is half that at 100% described above, and the motor rotates at ½ the maximum speed.
During the period when FET Q1 is ON, which exceeds the period during which FET Q4 is ON, that is, during the period when only FET Q1 is ON and FET Q4 is OFF, the power source current I2 from the motor coil L passes through the parasite diode of FET Q2 (or Q1) and is absorbed by the electrolytic capacitor group 31 (4 electrolytic capacitors EC) (see broken arrow I2). The electric charges absorbed by the electrolytic capacitor group 31 are discharged when FET Q2 is on. At that time, the diode D1 prevents the power source current from the motor coil L from flowing on the side of the direct current power source +B (reverse current prevention).
In case high level gate signals GS2 and GS3 and low level gate signals GS1 and GS4 are output respectively, to FETs Q2 and Q3 and to FETs Q1 and Q4, FETs Q2 and Q3 are set to ON, and the motor coil L is energized from the right end to the left end in the figure. Accordingly, FETs Q2 and Q3 energize the motor coil L with a duty ratio based on the gate signals GS2 and GS3, rotating the motor in reverse.
As described above, when the duty ratio is set to less than 100%, a period during which FET Q1 (or Q2) only is ON and FET Q4 (or Q3) is OFF occurs. During this period, the power source current I2 from the motor coil L passes through the parasite diode of FET Q2 (or Q1) and is absorbed by the electrolytic capacitor group 31 (see broken arrow 12). However, the problem associated with this occurrence was that a great amount of heat was generated from FET Q2 (or Q1), especially from its parasite diode.
In addition, when the electrolytic capacitor group 31 fails, for example when one of the electrolytic capacitor EC is short-circuited, a destruction of the circuit occurs. Each electrolytic capacitor EC constituting the electrolytic capacitor group 31 has a short lifetime compared to other components, and failures are easily provoked. In addition, since the electrolytic capacitor group 31 has a short lifetime, this would cause the lifetime of the device circuit to be shortened.
There was an additional problem that, compared to other components, the electrolytic capacitors EC occupy a large space on the component mounting printed board (not shown). Consequently, the electrolytic capacitor group 31 that is formed from these electrolytic capacitors EC occupies a significant amount of space on the printed board.
In addition, when such large space occupying electrolytic capacitors EC or electrolytic capacitor group 31 is mounted together with FETs Q1-Q4 on a printed board with a limited size, dissipation of the heat from FETs Q1-Q4 is prevented. Therefore, in order to increase the efficiency of heat dissipation, radiators, or fan motors, for example, had to be mounted increasing the cost significantly.